1. Field of the Invention
Glass compositions relating more particularly to improved light absorptive glass materials for use in fiber optical image-transfer devices.
2. Description of Prior Art
Fiber optical image-transfer devices such as are shown in application Ser. No. 238,372 now U.S. Pat. No. 3,387,959, entitled Fiber Optical Image Transfer Devices and Method of Making the Same, filed Nov. 19, 1962 and assigned to the same assignee as the present invention, have light-conducting optical fibers secured in bundled side-by-side relation with each other. Each optical fiber comprises a core filament, frequently formed of a lanthanum borate glass material surrounded by a cladding of a different glass material. The core filament material is of relatively higher refractive index than the fiber cladding material. Interspersed within this bundle between the light-conducting optical fibers are light-absorbing fibers each having a core filament of absorptive glass material surrounded by a cladding of a different light-transmitting glass material. Usually the same material is used for cladding both the light-conducting and light-absorbing fibers so that the claddings of both types of fibers can be easily fused together to form a vacuum-tight bundle of fibers. It will be understood that the core filaments and fiber cladding materials must be of compatible fusing temperature, drawing temperature, viscosity, coefficient of thermal expansion and the like so that the filament cores can be clad by conventional techniques and so that the fibers can be readily fused together. Particularly where the light-conducting fibers are to be drawn together with light-absorbing fibers to form multifiber units, it is desirable that the cores of both types of fibers be of compatible fusing temperature, drawing temperature etc. It will also be understood that, in order to perform their function as described in said copending application, the light-absorbing core materials are preferably adapted for extremely high absorption of incident light.